To Applanate or to Rebound? A preliminary comparison of the Tonopen and Tonovet tonometers

INTRODUCTION: For many years the Tonopen has been the standard, hand-held tonometer for veterinary use, providing an electronic measurement of intraocular pressure widely accepted throughout veterinary ophthalmology. It has some shortcomings, particularly when used in small eyes such as those of laboratory rodents, where the footplate is just too wide for accurate tonometry. More than 60 years ago, Obbink invented a totally different concept in tonometry,1 where a small probe is fired at the corneal surface and its rebound velocity is
measured, this correlating with intraocular pressure. Dekking improved the technique thirty years later 2 but it was not brought into widespread use until the beginning of this decade when Kontiola and colleagues developed a system for use in mice.3 The device has been marketed for the veterinary market as Tonovet. Comparison of this rebound or dynamic tonometer with the Tonopen applanation tonometer has been reported in the literature but only to date investigating animals with normotensive eyes. We have undertaken both in
vitro and in vivo studies of the device but here we provide some preliminary in vivo evidence comparing the two tonometers in both normotensive and glaucomatous canine eyes.

MATERIALS AND METHODS: 50 dogs of varying breeds with and without ocular pathology were included in the preliminary study. Dogs were examined ophthalmologically with direct and indirect ophthalmoscopy and slit lamp biomicroscopy. Measurements of intraocular pressure were performed with both tonometers. Topical anaesthetic (Amethocaine Minims, Chauvin) was applied to each eye 30 seconds prior to measurement with the Tonopen tonometer. Order of use of the Tonovet rebound tonometer and the Tonopen applanation tonometer was assigned randomly.

RESULTS: Four measurements were taken with each tonometer and the mean value was used as the intraocular pressure measurement for that eye. This mean value for intraocular pressure as measured with the Tonovet was plotted against the averaged readings for to Tonopen for each eye. In 12 dogs the mean value for one or other tonometer has a standard deviation greater than 10%, normally associated with a less than placid temperament. There was good correlation between the readings obtained with the Tonovet rebound tonometer and the Tonopen applanation tonometer (r=0.96), the Tonovet apparently over-estimated the intraocular pressure in severely glaucomatous eyes by around 2mmHg at 30mmHg, 5mmHg at 50mmHg and 7mmHg at 70mmHg.

DISCUSSION: Both tonometers were easy to use; the Tonovet had the advantage of not requiring local anaesthetic but the disadvantage of requiring horizontal use while the Tonopen can be used in any position. The good correlation between the two tonometers is encouraging although, as can be seen from figure 2 for each individual set of readings there can be significant variation between mean values. The higher values for the Tonovet compared with the Tonopen, especially in glaucomatous eyes is to be noted, although it is, of course, impossible from these readings to say which tonometer is providing the more accurate readings compared with the true pressure within the eye. We have performed experiments comparing Tonovet and Tonopen pressure readings in post-mortem eyes with the true intraocular pressure as determined by a U-tube manometer and will present these at a future meeting. This study shows that the Tonovet tonometer is a valuable tool in veterinary ophthalmology, useful for measuring the intraocular pressure in the canine eye.